Cathay vs mainland China

Cathay and mainland China both are nouns.

Word	Noun	Adjective	Verb	Adverb
Cathay	Yes	No	No	No
mainland China	Yes	No	No	No

As nouns, Cathay and mainland China are synonyms defined as:

Cathay and mainland China: a communist nation that covers a vast territory in eastern Asia; the most populous country in the world

Other synonyms of Cathay include China, Communist China, People's Republic of China, PRC, Red China.

Cathay (noun)	mainland China (noun)
a communist nation that covers a vast territory in eastern Asia; the most populous country in the world	a communist nation that covers a vast territory in eastern Asia; the most populous country in the world